1. Field of the Invention
The invention concerns an infrared light detector, a method to produce the infrared light detector and the use of the infrared light detector as a sensor to detect gas.
2. Description of the Prior Art
To detect gas it is known to use infrared light that lies within a wavelength range from 3 to 5 μm. The gas to be detected is exposed with a light source that is suitable to emit the infrared light in this wavelength range, wherein a portion of the infrared light is absorbed by the gas so that a residual light remains. The residual light can be measured with an infrared light sensor with which the wavelength range and/or the intensity of the residual light can be detected and can correspondingly be transduced into an electrical signal. Given a suitable evaluation the type and the content of the detected gas can be concluded on the base of the electrical signal.
A conventional infrared light sensor 101 is shown in FIG. 4, which infrared light sensor 101 has a substrate 102 that is obtained from a silicon wafer, for example. Multiple sensor chips 103 are mounted on the silicon wafer in a grid arrangement. The infrared light sensor 101 also has a pot-like TO housing 104 that covers the substrate 102 with the sensor chips 103 so that the sensor chips 103 are sealed from the environment with the TO housing 104. A filter 105 that is fashioned as an infrared light filter is installed in the top side of the TO housing 104 above every sensor chip 102.
According to FIG. 4, three sensor chips 103 with their filters 105 are provided in the infrared light sensor 101, wherein a left sensor chip filter pair, a middle sensor chip filter pair and a right sensor chip filter pair are shown.
The left filter 105 is set up such that it masks light with that wavelength that is absorbed by a gas type A. The middle filter 105 is designed for a gas type B and the right filter 105 is designed for a gas type C. If the gas to be detected includes the gas type A and strikes the residual light of the gas to be detected at the TO housing 104, the residual light passes through the filter 105 inside the TO housing 104. No light portion is filtered out of the residual light by the left filter 105, in contrast to which a corresponding light portion (component) is filtered out by the middle filter 105 and the right filter 105. The residual light that passed through the left filter 105 thus strikes the left sensor chip 103 that is then fully activated. In contrast to this, the middle sensor chip 103 and the right sensor chip 103 do not fully activate since the light that passed through the filter 105 associated with it is attenuated correspondingly by the filter effect of the filter 105. A detection of the gas type A results from the individual activation of the sensor chips 103.
The residual light passing through the left filter 105 is shown demarcated by a first light ray 106 and a second light ray 107 in FIG. 4. Due to a scatter effect of the left filter 105 on the residual light passing through it and/or due to a system-dependent variation of the gas detector system, the light rays 106 and 107 form a light bundle fanning out from the left filter 105 to the sensor chips 103. An exposure surface enclosed by the light rays 106 and 107 results from this on the substrate 102. It is desirable that the right sensor chip 103 and the middle sensor chip 103 lie outside of the exposure area since an illumination of the middle sensor chip 103 (for example) as is shown in FIG. 4 can lead to misinterpreted activation of the sensor chips 103. The TO housing 104 is conventionally made of metal. Depending on the manufacture, the height of the TO housing is 1 to 2 mm. A necessary minimum distance of 0.1 to 1 mm results from this between the individual sensor chips 103 so that, for example, the middle sensor chip 103 lies outside of the exposure area of the left sensor chip 103. A large overall width of the arrangement of the sensor chips 103 results from this, so the dimensions of the substrate 102 of corresponding size must be provided. The infrared light sensor 101 could alternatively be equipped with only one sensor chip filter pair, such that multiple such infrared light sensors 101 would be necessary to detect multiple gas types. However, the cost of such multiple infrared light sensors 101 would be too high.